A packaging laminate of the above-described type is previously known from, for example, international patent application carrying publication number WO 97/02140. The known packaging laminate has a rigid, but foldable core layer of paper or paperboard and outer, liquid-tight coatings of humidity and heat-resistant thermoplastic material on both sides of the core layer. In order to impart to the known packaging laminate tightness properties also against gases, in particular oxygen gas, the packaging laminate moreover has a gas barrier, for example an aluminium foil (Alifoil) disposed between the core layer and the one outer coating.
From the known packaging laminate, retortable packaging containers or cartons are produced with the aid of filling machines of the type which, from a web or from prefabricated blanks of the packaging laminate, form, fill and seal finished packages according to the so-called form/fill/seal technology.
From, for example, a flat-folded tubular packaging blank of the known packaging laminate, retortable packaging containers are produced in that the packaging blank is first raised to an open, tubular packaging carton which is sealed at its one end by fold-forming and sealing of the continuous, united foldable end panels of the packaging carton, for the formation of a substantially planar bottom closure. The packaging carton provided with the bottom is filled with the relevant contents, e.g. a food, through its open end which is thereafter closed by a further fold-forming and sealing of the opposing end panels of the packaging carton for the formation of a substantially planar top closure. The filled and sealed, normally parallelepipedic packaging container is thereafter ready for heat treatment in order to impart to the packed food extended shelf life in the unopened packaging container.
A shelf life extending heat treatment of the packed food may suitably be put into effect in the manner and under the conditions described more closely in international patent application carrying publication number WO 98/16431, which is hereby embodied as a reference. The packaging container is placed in a retort and heated therein with the aid of a first circulating gaseous medium, e.g. hot steam, to a temperature which in general lies in the range of between 70 and 130° C. After a predetermined stay time at this selected temperature, the supply of the first gaseous medium is discontinued. The packaging container is thereafter cooled with a second circulating gaseous medium, e.g. cold air, and finally with a circulating liquid medium, e.g. cold water. The cooled packaging container is thereafter removed from the retort for further transport and handling. The total treatment time, including the time for heating and the time for cooling from the selected treatment temperature should be sufficient to give, in each individual case, the relevant food a desired combination of high Fo-value and low Co-value. The expressions “Fo-value” and “Co-value” are known to persons skilled in the art and relate to the time (mm) which the food would need to be heated at a reference temperature (121° C.) in order to achieve the same level of sterility, and the time the food would need to be heated at a reference temperature (100° C.) in order to achieve the same level of cooking effect on all of the component ingredients in the food, respectively. To a person skilled in the art, it will be obvious that a higher treatment temperature in the retorting process gives a higher Fo-value and a lower Co-value than a lower treatment temperature in a corresponding retorting during the same total treatment time, and that a retorting of the packed food consequently should be carried out at a relatively high treatment temperature within the range of 70–90° C. in order to achieve the desired combination of high Fo-value and low Co-value.
A packaging container of the known packaging laminate generally functions well in a normal heat treatment in a retort, but, on the other hand, problems may not seldom arise in such cases when the heat treatment is carried out at an extremely high treatment temperature within the range of 70–130° C. and/or during an extremely long treatment time, even at a relatively low treatment temperature within this range. For example, it has proved that the internal bonding strength between the layers included in the packaging laminate have a tendency to become weakened, and that this weakening may even be so great that the packaging laminate, at excessively high treatment temperatures, delaminates, whereby the packaging container loses both its mechanical strength and configurational stability, as well as its desired tightness properties.